The primary structural components of an offshore petroleum recovery installation are (1) the pilings which are driven into the sea bed to form the foundation for the structure, (2) the platform which acts as a template to structurally interconnect the pilings and provides support for various subsea equipment associated with the well, and (3) the decking which is supported above the water line on the main pilings. Those familiar with the design of offshore petroleum recovery structures recognize that the subsea platform typically consists of inclined main jacket legs, comparatively short satellite jacket legs, and cross braces which interconnect the legs to provide a unitary platform. The main pilings generally extend from above the water line through the respective main platform legs and into the sea bed, while satellite pilings typically extend from just above the shorter jacket legs into the sea bed. Significant static and dynamic axial, radial and rotational forces are transmitted from each piling to its surrounding leg, and from each leg to its interiorly positioned piling. The platform serves to transfer some of this stress load from the main pilings to the satellite pilings, and acts to more uniformly distribute the remaining load over each of the main pilings.
In order to provide the desired structural support between the pilings, it is essential that the platform legs be securely affixed to the respective piling within that leg. The desired structural connection is most commonly obtained by providing a pair of spaced inflatable members in the annulus between the platform leg and the piling, and then filling the annulus between the spaced inflatable members within concrete or other grouting material. Cured grouting in this annulus thus provides the necessary interconnection between the platform legs and each interiorly positioned piling to withstand the substantial axial, radial and rotational forces acting between a platform leg and a piling, and thus cooperates with the platform to provide the desired structural interconnection between the various pilings of the offshore petroleum recovery installation.
In spite of the widespread acceptance of the above-described grouting technique, this procedure has significant drawbacks. The cylindrical-shaped piling and external platform tubulars are rarely concentric, and it is thus difficult and expensive to ensure that grouting has adequately filled thinner annulus spacings between each platform leg and its eccentrically positioned piling. Most importantly, however, is the fact that a subsea platform cannot practically be separated from its pilings during subsequent salvage operations, i.e., when the economic life of the offshore wells serviced by the installation has been exhausted. While the pilings themselves are generally metallic tubulars which can be cut below the sea bed mud line, the platform cannot thereafter be practically refloated to the surface because of the substantial weight which the pilings and the grouting add to the platform. As a consequence, platforms secured to pilings by the above-described grouting technique are customarily cut up subsea, and individual components then retrieved to the surface. This subsea disassembly of the platform is not only extremely expensive, but also substantially destroys the value of the platform for subsequent use. Those skilled in the art recognize that millions of dollars are lost annually because subsea petroleum recovery platforms grouted to the pilings cannot be practically recovered.
Although the above-described grouting technique is used worldwide in most petroleum recovery installations, platform legs have been mechanically connected to pilings by expanding the piling radially outward into fixed engagement with the platform legs. Equipment and techniques for accomplishing this radial expansion of the pilings are marketed under the trade names "Lynes Corrigator" and "Hydrolock". This radial expansion technique has not been widely accepted, however, both because of its expense and because the process of forming the mechanical interconnection between the piling and the platform leg inherently reduces the structural integrity of the piling. Moreover, this technique generally does not allow the platform as a unitary structure to be refloated to the surface, since the substantial weight of the interconnected pilings renders floatation impracticle, and since the formed interconnection between the pilings and the platform legs cannot be practically disengaged.
The disadvantages of the prior art are overcome by the present invention, and improved techniques are hereinafter disclosed for connecting jacket legs of an offshore platform to interiorly positioned cylindrical pilings.